I received your example. I have modified it a little bit.
Since loading data with the property ‘ virtualized’ of the list box significantly increases the performance I have implemented a drop-down list with the combination of a drop-down button and a list box. Also, I have applied some CSS as you can see in the ‘elementi.css’.
You can download a copy of the modified version from here: https://easyupload.io/0eeudv
See how a managed to do the drop-down list and you can implement it with every drop-down list which has got many data.

Please use this method only when you have got many data. For a drop-down with a few list items use the default drop-down list.

See ‘elementi.css’, ‘index.html’, ‘combobox.js’

I hope this helps!

Best Regards,
Svetoslav Borislavov

Smart UI Team